Monoclonal antibodies (MAb) directed to murine cerebral vascular endothelial cells (EC) were produced, purified and characterized. Murine cerebral vascular EC isolated from 3-5 day old mice (SJL/J) were cultivated and used to characterize the MAb. Cell populations which stained positively for EC marker (Factor VIII antibody) were observed to differ in their staining pattern for the various preparations of MAb. Recent results indicate that the MAb significantly differ in their staining pattern. Some selectively stained murine cerebral vascular EC while others demonstrated cross reactivity with other cells. The MAb staining pattern suggested that EC preparations differ in respect to their surface antigen expression. The ability of cultured cells to stain for Factor VIII decreased in relation to length of culture while some MAb staining remained unchanged indicating differential regulation of these antigens.